The present invention relates to a spark plug for use in an internal combustion engine.
A certain conventional spark plug includes a center electrode which is disposed in an insulator in such a manner as to project from the front end face of the insulator, and a parallel ground electrode whose one end is disposed in parallel with the end face of the center electrode and whose other end is joined to a metallic shell, and the spark plug is adapted to ignite a fuel mixture gas through spark discharge across a gap between the center electrode and the parallel ground electrode.
In order to enhance an ignition property for igniting a fuel mixture gas, Japanese Patent Application Laid-Open (kokai) Nos. 5-326107 and 7-130454 propose a spark plug which includes, in addition to a ground electrode which faces the end face of a center electrode in parallel, auxiliary ground electrodes whose end faces face the circumferential side surface of the center electrode. An object of disposing the auxiliary ground electrodes is not to induce sparking across the gap between the end face of an auxiliary ground electrode and the circumferential side surface of the center electrode, but to improve distribution of electric field between the parallel ground electrode and the center electrode so as to induce sparking between the parallel ground electrode and the center electrode at a lower discharge voltage, thereby enhance ignition characteristics. The structural design of the proposed spark plugs is not intended to bring an edge of the end face of an auxiliary ground electrode in the vicinity of the front end face of an insulator.
Japanese Patent Application Laid-Open (kokai) No. 9-199260 proposes a spark plug that includes, in addition to a parallel ground electrode which faces the end face of a center electrode, auxiliary ground electrodes provided in the vicinity of the end face of an insulator.
However, the above-mentioned spark plugs described in Japanese Patent Application Laid-Open (kokai) Nos. 5-326107 and 7-130454 involve a problem in that spark discharge tends to fail to occur at a predetermined position, upon occurrence of so-called xe2x80x9ccarbon fouling.xe2x80x9d At the time of regular operation, in which an internal combustion engine is operating at a predetermined temperature and at a predetermined rotational speed or higher, the temperature of a leg portion of an insulator of a spark plug increases to an appropriate level, and the surface temperature as measured in the vicinity of the end face of the insulator located within a combustion chamber increases to about 500xc2x0 C. At such a temperature, carbon adhering to the surface of the insulator is burnt out, so that the insulator surface is held clean. Therefore, a problem associated with xe2x80x9ccarbon foulingxe2x80x9d does not arise. By contrast, at the time of low-load operation, in which the internal combustion engine is operating at low rotational speed and at extremely low temperature, the surface temperature of the insulator does not increase, and thus carbon accumulatively adheres to the surface of the insulator; i.e., the xe2x80x9ccarbon foulingxe2x80x9d state is established. When xe2x80x9ccarbon foulingxe2x80x9d progresses, insulation between the center electrode and the ground electrode is impaired; as a result, spark discharge is disabled, leading to engine stall.
The above-mentioned Japanese Patent Application Laid-Open (kokai) No. 9-199260 does not specify the relationships among the distance between the parallel ground electrode and the center electrode (air gap), the distance between an auxiliary ground electrode and the center electrode (semi-creepage gap), and the distance between the end face of an auxiliary ground electrode and the circumferential side surface of the insulator (insulator gap).
Japanese Patent Application Laid-Open (kokai) No. 59-71279 discloses a semi-creepage discharge spark plug configured such that a ground electrode is disposed in opposition to the circumferential side surface of an insulator. In the semi-creepage discharge spark plug, a spark creeps along the surface of the insulator, and thus carbon adhering to the insulator surface is burnt out. Therefore, xe2x80x9ccarbon foulingxe2x80x9d does not raise a serious problem. However, since a spark continuously creeps along the surface of the insulator, the insulator surface is damaged by sparks; i.e., a so-called xe2x80x9cchannelingxe2x80x9d problem arises, shortening the life of the spark plug.
An object of the present invention is to provide a spark plug which is less affected by xe2x80x9ccarbon fouling,xe2x80x9d exhibits excellent ignition characteristics, and reduces the amount of channeling.
To achieve the above object, a spark plug of the present invention assumes the following basic structure. Specifically, the spark plug of the present invention comprises an insulator having a center through-hole formed therein; a center electrode held in the center through-hole and disposed at an end portion of the insulator; a metallic shell for holding the insulator such that an end portion of the insulator projects from an end face thereof; a parallel ground electrode disposed such that one end thereof is joined to a front end face of the metallic shell, and a side face of the other end faces, in parallel, an end face of the center electrode; and a plurality of semi-creeping discharge ground electrodes disposed such that one end of each of the electrodes is joined to the metallic shell, and the other end of each of the electrodes faces a circumferential side surface of the center electrode and/or a circumferential side surface of the insulator. In the spark plug of the present invention, an air gap (xcex1) is formed between the parallel ground electrode and the end face of the center electrode; a semi-creepage gap (xcex2) is formed between the end face of each of the semi-creeping discharge ground electrodes and the circumferential side surface of the center electrode; an insulator gap (xcex3) is formed between the end face of each of the semi-creeping discharge ground electrodes and the circumferential side surface of the insulator; a distance xcex1 across the air gap (xcex1) and a distance xcex2 across the semi-creepage gap (xcex2) satisfy the relationship xe2x80x9cxcex1 less than xcex2;xe2x80x9d and the distance xcex1 across the air gap (xcex1) and a distance xcex3 across the insulator gap (xcex3) satisfy the relationship xe2x80x9cxcex1 greater than xcex3.xe2x80x9d Notably, herein, reference symbols (xcex1), (xcex2), and (xcex3) for denoting gaps as structural elements of the invention may also be used to denote the sizes of gaps. In this connection, the following convention for usage of reference symbols may be adopted: reference symbols Gxcex1, Gxcex2, and Gxcex3 are used to denote gaps as structural elements, whereas reference symbols xcex1, xcex2, and xcex3 are used to denote the sizes of gaps. However, herein, in order to avoid complication of description, the same reference symbols are used to denote gaps as structural elements and the sizes of the gaps.
Satisfaction of the above-described relationships yields the following advantages. Since the distance xcex1 across the air gap (xcex1) is shorter than the distance xcex2 across the semi-creepage gap (xcex2) (xcex1 less than xcex2), in the normal state; i.e., when the xe2x80x9ccarbon foulingxe2x80x9d state is not established, spark discharge occurs across the air gap (xcex1) associated with the parallel ground electrode. Since the distance xcex3 across the insulator gap (xcex3) is shorter than the distance xcex1 across the air gap (xcex1), when the front end face of the insulator is fouled with carbon to thereby enter the xe2x80x9ccarbon foulingxe2x80x9d state, spark discharge called semi-creeping discharge occurs along the end face of the insulator between an edge of the end face of the semi-creeping discharge ground electrode and the circumferential side surface of the center electrode. A spark associated with semi-creeping discharge runs across the insulator gap (xcex3) and then along the surface of the insulator (along the reverse route when the voltage polarity is inverted). When semi-creeping discharge is repeated several times, carbon deposited on the front end face of the insulator is burnt out, and thus the surface of the insulator is restored to a clean state, thereby restoring insulation on the surface of the insulator and eliminating xe2x80x9ccarbon fouling.xe2x80x9d As a result, the site of spark discharge returns to the air gap (xcex1) from the semi-creepage gap (xcex2). Notably, as shown in FIG. 2, the distance xcex2 across the semi-creepage gap (xcex2) appearing herein means the minimum distance between the semi-creeping discharge ground electrode and the circumferential side surface of the center electrode located on the same plane as that of the front end face of the insulator, as measured along the direction perpendicular to the axis of the spark plug. The distance xcex3 across the insulator gap (xcex3) means the minimum distance between the insulator and the semi-creeping discharge ground electrode.
In a spark plug having the above-described basic structure, spark discharge occurs mostly across the air gap (xcex1) associated with the parallel ground electrode. Only when the surface of the insulator is fouled with carbon; i.e., only in the xe2x80x9ccarbon foulingxe2x80x9d state, semi-creeping discharge occurs across the semi-creepage gap (xcex2) associated with the semi-creeping discharge ground electrode, thereby igniting a fuel mixture gas in a combustion chamber. In such a spark plug, a fuel mixture gas is ignited mostly through spark discharge across the air gap (xcex1), and thus the spark plug exhibits excellent ignition characteristics. Since the spark plug can perform a self-cleaning action; i.e., can burn out carbon deposited on the surface of the insulator through semi-creeping discharge, the spark plug can readily cope with xe2x80x9ccarbon fouling.xe2x80x9d Further, the frequency of semi-creeping discharge decreases, and semi-creeping discharge ends within a very short period of time. Therefore, the amount of xe2x80x9cchannelingxe2x80x9d induced by spark decreases considerably, and channeling rarely occurs, thereby sufficiently extending the life of the spark plug.
As shown in FIG. 23, when a spark plug (100) of the present invention having two semi-creeping discharge ground electrodes (12, 12) is to be mounted on a direct-injection-type internal combustion engine (150), preferably the semi-creeping discharge ground electrodes (12, 12) are directed to an intermediate region between intake valves (201) and exhaust valves (203).
In the example of FIG. 23, when there are assumed a virtual reference plane (SP) including a center axis (O) of the spark plug (100) mounted on a cylinder head (S) and a virtual auxiliary reference plane (CSP) including the center axis (O) and perpendicularly intersecting the reference plane (SP), the intake valves (201, 201) are disposed on one side with respect to the reference plane (SP), and the exhaust valves (203, 203) are disposed on the opposite side with respect to the reference plane (SP), such that all the valves are located substantially the same distance from the reference plane (SP). One intake valve (201) and one exhaust valve (203) are disposed on one side with respect to the auxiliary reference plane (CSP), and the other intake valve (201) and the other intake valve (203) are disposed on the opposite side with respect to the auxiliary reference plane (CSP). The semi-creeping discharge ground electrode (12) is disposed such that its base end attached to a metallic shell (5) is located closer to the reference plane (SP) than to the auxiliary reference plane (CSP); in FIG. 23, the base end is located substantially on the reference plane (SP). A parallel ground electrode (11) is disposed such that its base end attached to the metallic shell (5) is located closer to the auxiliary reference plane (CSP) than to the reference plane (SP); in FIG. 23, the base end is located substantially on the auxiliary reference plane (CSP).
The above-described spark plug (100) differs in mounting orientation from an ordinary spark plug having a parallel ground electrode only. In a combustion chamber (CR) of the internal combustion engine (150), an intake air discharged from the intake valve (201) flows toward the exhaust valve (203). In this connection, according to a study conducted by the present inventors, mounting orientation of the spark plug (100) of the present invention to be applied to a direct-injection-type internal combustion engine must be determined in consideration of a vertical flow (tumble), which is induced by a cavity formed on a piston (P) biasedly extending from a central portion of the piston (P) to a side toward the intake valve, and a horizontal flow (squish), which arises from rise of the piston (P) and is directed toward the cavity from a region along the wall surface of the combustion chamber (CR); more specifically, the mounted spark plug (100) must be oriented such that the semi-creeping discharge ground electrodes (12) reliably exhibit ignition characteristics thereof. When the spark plug (100) is mounted in such a manner as to establish the above-described positional relationships, a spark generated from either of the semi-creeping discharge ground electrodes (12) is directed substantially perpendicular to a flow of intake air, although the spark is likely to be influenced by squish, in view of the semi-creeping discharge ground electrode (12) being located near the wall surface of the combustion chamber.
Particularly, in the spark plug (100) having two semi-creeping discharge electrodes (12, 12) located at opposite positions offset 90xc2x0 from the parallel ground electrode (11), orienting a welded portion between the parallel ground electrode (11) and the metallic shell (5) toward a side facing the intake valve (201) is particularly effective. In other words, orienting the free end of the parallel ground electrode (11) toward a side facing the exhaust valve (203) is preferred. Since a spark generated from the parallel ground electrode (11) is influenced by both tumble and squish, the spark encounters a flow of intake air coming from a position obliquely forward of the spark plug. Since the flow of intake mixture is of a considerably high flow rate, if the parallel ground electrode (11) is oriented in reverse with respect to the orientation described above, a spark drifts away from the parallel ground electrode (11) during the course of sparking, potentially resulting in interruption of sparking. Through employment of the above-described orientation, even when a spark drifts, interruption of sparking becomes unlikely, by virtue of presence of the parallel ground electrode (11), and thus impairment in ignition characteristics becomes unlikely.
In an internal combustion engine having two intake valves (201, 201) and two exhaust valves (203, 203) (i.e., in a 4-valve internal combustion engine), the above-described arrangement may be employed, while the intake valves (201, 201) and the exhaust valves (203, 203) are handled in pairs. Specifically, the following arrangement may be employed. Generally, in a 4-valve internal combustion engine, two intake valves (201, 201) are disposed on one side of a pent-roof-type cylinder head (S), which assumes a miter roof shape as viewed from the front side of the internal combustion engine, (i.e., on one side with respect to the reference plane (SP)), and two exhaust valves (203, 203) are disposed on the opposite side. The intake valve (201) and the exhaust valve (203) that are located on the same side with respect to the auxiliary reference plane (CSP) face each other with the reference plane (SP) being interposed therebetween. The spark plug (100) may be mounted such that the semi-creeping discharge ground electrodes (12, 12) are located at intermediate angular positions about the center axis (O) between the paired, mutually facing intake and exhaust valves (201, 203).
The present inventors studied a spark plug having the basic structure described above and found that the position of sparking is not determined by solely the distance between electrodes, and that under certain conditions sparking occurs even across a large gap (herein called xe2x80x9cinverse sparking phenomenonxe2x80x9d). The inverse sparking phenomenon raises a problem in that upon occurrence of xe2x80x9ccarbon fouling,xe2x80x9d sparking does not occur across the insulator gap (xcex3) as expected, but occurs between the insulator and the front end face of the metallic shell (herein called xe2x80x9cmetallic-shell-insulator sparkingxe2x80x9d). Several configurations of a spark plug according to the present invention provide specific means for solving problems introduced by the inverse sparking phenomenon, such as metallic-shell-insulator sparking.
Prevention of, for example, metallic-shell-insulator sparking is greatly effective in the case of a direct-injection-type internal combustion engine of stratified-charge combustion system. In a direct-injection-type internal combustion engine, impairment in ignition characteristics is likely to result from sparking between the insulator and the front end face of the metallic shell, implying that the position of sparking influences ignition characteristics. Specifically, in an internal combustion engine of stratified-charge combustion system, a rich-mixture layer is present in a very narrow region within a combustion chamber. In the remaining region, the mixture becomes considerably lean. Whether or not a spark can be reliably generated in the rich-mixture layer determines whether or not the mixture is ignited normally. If a spark can be reliably generated across the regular spark discharge gap of a spark plug; i.e., across the gap between the center electrode and the ground electrode, upon arrival of the rich-mixture layer at the gap, the spark can ignite intake mixture.
However, as mentioned above, the rich-mixture layer is formed only in a very narrow region. Thus, if a spark is not generated across the regular spark discharge gap, but is generated at a different position (i.e., near the wall surface of the combustion chamber) as in the case of metallic-shell-insulator sparking, the mixture is not ignited; i.e., misfire occurs, in spite of generation of spark, since the mixture is very lean at that position. Such sparking in the vicinity of the wall surface of the combustion chamber or at a like position results in misfire in the combustion cycle, resulting in a drop in the output of the internal combustion engine and a potential failure to satisfy the emission regulations due to ejection of unburnt mixture from an exhaust pipe. Further, an unburnt gas is not completely exhausted from the exhaust pipe, but adheres to the wall surface of the combustion chamber and to the spark plug. As a result, the insulator is wetted with fuel, and thus sparking becomes more difficult in the next cycle.
Thus, if the inverse sparking phenomenon and associated metallic-shell-insulator sparking are prevented to thereby reliably induce sparking across the air gap (xcex1) or the semi-creepage gap (xcex2), even when xe2x80x9ccarbon foulingxe2x80x9d occurs, deposited carbon can be burnt out through sparking by the semi-creeping discharge ground electrode. Even in a direct-injection-type internal combustion engine, if sparking is induced by the semi-creeping discharge ground electrode, impairment in ignition characteristics can be suppressed, since rich mixture is exposed to sparking. That is, the present invention is applicable not only to an ordinary internal combustion engine, but also to a direct-injection-type internal combustion engine. Particularly, application to a direct-injection-type internal combustion engine is greatly effective in suppressing xe2x80x9ccarbon fouling,xe2x80x9d reducing the amount of xe2x80x9cchanneling,xe2x80x9d and reducing ablation of the center electrode on the side surface thereof. However, the conventional art as described in the aforementioned Japanese Patent Application Laid-Open (kokai) No. 9-199260, among others, fails to propose improvement in a spark plug from the above-described point of view.
On the assumption that the above-described basic structure is employed, configurations of a spark plug of the present invention will next be described in more detail.
A first configuration is characterized by assuming the above-described basic structure, and in that:
the air gap (xcex1) is not greater than 1.1 mm (1-(i));
the insulator gap (xcex3) falls within a range of 0.5 mm to 0.7 mm (1-(ii)); and
a diametral difference xcex4 between the insulator and the metallic shell as measured along the front end face of the metallic shell is not less than 3.6 mm (1-(iii)).
The present inventors carried out extensive studies and experimentally proved that, in a spark plug having the above-described basic structure, satisfaction of the above-described conditions (1-(i)) to (1-(iii)) in relation to the air gap (xcex1), the insulator gap (xcex3), and the diametral difference (xcex4) between the insulator and the metallic shell effectively suppresses the inverse sparking phenomenon and associated metallic-shell-insulator sparking even upon occurrence of, for example, xe2x80x9ccarbon fouling,xe2x80x9d thereby ensuring sparking across the insulator gap (xcex3). On the basis of this finding, the invention of the first configuration was accomplished.
The air gap (xcex1) can be designed to various values according to required ignition characteristics, air-fuel ratio of mixture, and other factors. Since the insulator gap (xcex3) must be smaller than the air gap (xcex1), the insulator gap (xcex3) is set to a value falling within an appropriate range according to the air gap (xcex1). In a spark plug according to the first configuration, the air gap (xcex1) and the insulator gap (xcex3) are assumed to be set to values falling within the ranges (1-(i)) and (1-(ii)), respectively. The gist of the first configuration lies in that, under this assumption, the diametral difference xcex4 between the insulator and the metallic shell as measured along the front end face of the metallic shell is set to a value falling within the range (1-(iii)). By setting the diametral difference (xcex4) to such a value, even when xe2x80x9ccarbon foulingxe2x80x9d occurs, a foul substance adhering to the insulator can be burnt out through sparking induced by the semi-creeping discharge ground electrode. Even in a direct-injection-type internal combustion engine, if sparking is induced by the semi-creeping discharge ground electrode, impairment in ignition characteristics can be suppressed, since rich mixture is exposed to sparking. The distance xcex1 across the air gap (xcex1) cannot be reduced unconditionally. A distance xcex1 of, for example, not less than 0.6 mm is effective in view of attainment of required ignition characteristics, prevention of short circuit upon adhesion of electrically conductive foreign matter as in the case of, for example, fouling (the same also applies to spark plugs according to other configurations of the present invention). The diametral difference (xcex4) cannot be increased unconditionally. For example, a diametral difference (xcex4) not greater than 5.4 mm, preferably not greater than 5.0 mm, is effective (the same also applies to spark plugs according to other configurations of the present invention).
A spark plug according to a second configuration is characterized by assuming the previously described basic structure, and in that:
the air gap (xcex1) falls within a range of 0.8 mm to 1.0 mm (2-(i));
the insulator gap (xcex3) falls within a range of 0.5 mm to 0.7 mm (2-(ii)); and
the air gap (xcex1) and the insulator gap (xcex3) satisfy the relationship xe2x80x9c0.2 mmxe2x89xa6(xcex1xe2x88x92xcex3)xe2x89xa60.4 mmxe2x80x9d (2-(iii)). The invention of the second configuration can be combined with the invention of the first configuration.
In the spark plug of the second configuration, in order to reduce spark voltage, the air gap (xcex1) is set to a value falling within the somewhat narrower range (2-(i)), and the insulator gap (xcex3) is set to a value falling within the range (2-(ii)) (equal to that of the first configuration). The difference (xcex1xe2x88x92xcex3) between the air gap (xcex1) and the insulator gap (xcex3) is set to a value falling within the range (2-(iii)), thereby effectively suppressing the inverse sparking phenomenon and associated metallic-shell-insulator sparking. The second configuration yields the following new, additional effect. In a direct-injection-type internal combustion engine, the range of injection end timing in which no misfire occurs can be extended.
Generally, in an internal combustion engine, the greater the air gap (xcex1), the more desirable the ignition characteristics. However, discharge voltage increases with the air gap (xcex1). Since a direct-injection-type internal engine is considerably prone to xe2x80x9ccarbon fouling,xe2x80x9d even at the time of regular operation, xe2x80x9ccarbon foulingxe2x80x9d occurs. When the xe2x80x9ccarbon foulingxe2x80x9d state is established, high discharge voltage increases the possibility of misfire. In a direct-injection-type internal combustion engine, ignition characteristics are said to improve with a misfire-free range in relation to ignition timing for sparking a spark plug as represented by crank angle and fuel injection end timing as represented by crank angle.
In a direct-injection-type internal combustion engine, a rich-mixture region emerging immediately after injection diffuses within a combustion chamber while gradually moving within the chamber. Therefore, the following tendency arises: the earlier the fuel injection end timing, the greater the diffusion of the rich-mixture region, thereby becoming thin, when a spark plug is to spark. Thus, lean mixture must be ignited. Since mixture to be ignited is thin, discharge voltage increases although the spark gap remains unchanged. As mentioned above, since the spark plug is usually in the xe2x80x9ccarbon foulingxe2x80x9d state, carbon fouling and an increase in discharge voltage induced by lean mixture make the spark plug more prone to involve sparking between the metallic shell and the insulator; i.e., occurrence of metallic-shell-insulator sparking becomes more likely. As a result, the spark plug becomes prone to misfire. By contrast, the later the fuel injection end timing, the more likely sparking is to occur in rich mixture. In this state, combustion is carried out stably. However, in spite of stable combustion, sparking in rich mixture increases a tendency toward xe2x80x9ccarbon fouling.xe2x80x9d As a result, a spark is generated between the metallic shell and the insulator, potentially causing misfire.
The present inventors carried out studies and found the following: in a common internal combustion engine, the greater the air gap (xcex1), the more desirable the ignition characteristics, whereas, in a direct-injection-type internal combustion engine, discharge voltage increases with gap, with a resultant impairment in ignition characteristics. According to an invention of the second configuration, the air gap (xcex1) and the insulator gap (xcex3) are set to values falling within the range (2-(ii)), and the air gap (xcex1) and the insulator gap (xcex3) conform to the relationship (2-(ii)), thereby suppressing sparking between the insulator and the front end face of the metallic shell and thus widening the range of stable combustion.
Extending a stable combustion range is preferred for the following reason. In a direct-injection-type internal combustion engine, ignition timing and fuel injection timing are controlled according to operation conditions. However, for example, when the throttle opening is changed abruptly, control may become inconsistent with a change in atmosphere around the spark plug. In such a state, a transitional phenomenon such as deviation in fuel injection timing or ignition timing from desirable timing may cause mixture around the spark plug to become thin or thick. When there arises a tendency for an increase in the interval between fuel injection timing and ignition timing, mixture becomes thin, and thus discharge voltage increases. When there arises a tendency for a decrease in the interval between fuel injection timing and ignition timing, sparking occurs in thicker mixture, and thus carbon fouling is worsened. Therefore, use of a spark plug having a wide stable combustion range ensures good combustion without involvement of misfire, even when such a transitional phenomenon arises.
Preferably, the diameter of a front end portion of the center electrode is reduced; and the width W of the parallel ground electrode as viewed from an axially frontward side of the insulator and measured across the center point of the center electrode is not greater than 2.2 mm and is not less than two times the outside diameter of the center electrode as measured along the front end face of the center electrode. Employment of such dimensional relationship decreases discharge voltage and prevents occurrence of a so-called bridge, which is a problem such that fuel is trapped between the center electrode and the ground electrode, while maintaining ignition characteristics intact.
A third configuration is characterized by assuming the previously described basic structure, and in that:
the air gap (xcex1) is not greater than 0.9 mm (3-(i));
the insulator gap (xcex3) falls within a range of 0.5 mm to 0.7 mm (3-(ii)); and
the diametral difference xcex4 between the insulator and the metallic shell as measured along the front end face of the metallic shell is not less than 2.8 mm (3-(iii)). The third configuration can be combined with at least either the first configuration or the second configuration.
In a spark plug according to the third configuration, the air gap (xcex1) and the insulator gap (xcex3) are assumed to be set to values falling within the ranges (3-(i)) and (3-(ii)), respectively. The air gap (xcex1) is set to a value falling within a range narrower than the range (1-(i)) in the first configuration, for the same reason as in the case of the second configuration. Under this assumption, by setting the diametral difference (xcex4) between the insulator and the metallic shell as measured along the front end face of the metallic shell to a value falling within the range (3-(iii)), even when xe2x80x9ccarbon foulingxe2x80x9d occurs, a foul substance adhering to the insulator can be burnt out through sparking induced by the semi-creeping discharge ground electrode. Even in a direct-injection-type internal combustion engine, if sparking is induced by the semi-creeping discharge ground electrode, impairment in ignition characteristics can be suppressed, since rich mixture is exposed to sparking.
A fourth configuration is characterized by assuming the previously described basic structure, and in that:
the air gap (xcex1) is not greater than 1.1 mm (4-(i));
the insulator gap (xcex3) falls within a range of 0.5 mm to 0.7 mm (4-(ii)); and
three or more semi-creeping discharge ground electrodes are disposed (4-(iii)). The fourth configuration can be combined with at least any one of the first to third configurations.
In a spark plug according to the fourth configuration, the setting ranges (4-(i)) and (4-(ii)) for the air gap (xcex1) and the insulator gap (xcex3), respectively, are the same as (1-(i)) and (1-(ii)) in the first configuration. The fourth configuration differs from the first configuration in that three or more semi-creeping discharge ground electrodes are disposed in place of employing a requirement for the diametral difference (xcex4), in order to reduce frequency of occurrence of the inverse sparking phenomenon and associated metallic-shell-insulator sparking.
An increase in the number of semi-creeping discharge ground electrodes means an increase in the probability of occurrence of sparking from a semi-creeping discharge ground electrode. Accordingly, even when a spark plug is surrounded by an atmosphere which would cause metallic-shell-insulator sparking if the number of semi-creeping discharge ground electrodes is fewer, an increase in the number of semi-creeping discharge ground electrodes located in the vicinity of the front end face of the metallic shell allows reliable generation of spark by a semi-creeping discharge ground electrode upon occurrence of xe2x80x9ccarbon fouling,xe2x80x9d thereby burning out an adhering foul substance. Even in a direct-injection-type internal combustion engine, if sparking is induced by the semi-creeping discharge ground electrode, impairment in ignition characteristics can be suppressed, since rich mixture is exposed to sparking.
When a spark plug is attached to an internal combustion engine, a front end portion of an insulator is cooled by intake air, whose temperature is relatively low, introduced into a combustion chamber through an intake valve. As the number of semi-creeping discharge ground electrodes increases, a front end portion of the insulator is hidden behind semi-creeping discharge electrodes and thus may be less cooled, potentially inducing preignition. In view of this, preferably the number of semi-creeping discharge ground electrodes to be disposed is not greater than 4. The fourth configuration can be configured so as to satisfy the requirement (1-(iii)) for the diametral difference xcex4 in the first configuration.
A spark plug according to a fifth configuration is characterized by assuming the previously described basic structure, and in that:
a front end portion of the insulator is formed into a straight tubular portion; with the term xe2x80x9cfrontwardxe2x80x9d referring to a side toward the front end portion of the insulator along the axial direction of the insulator, a rear edge of the end face of the semi-creeping discharge ground electrode is aligned with or is located frontward of the rear end position of the straight tubular portion; and the level difference E (unit: mm) along the axial direction between the front end face of the insulator and the rear edge of the end face of the semi-creeping discharge ground electrode, and the radius of curvature R (unit: mm) of a curved surface extending from the front end face of the insulator to the circumferential side surface of the insulator satisfy the relationship indicative of a difference therebetween xe2x80x9cRxe2x88x92Exe2x89xa60.1 mmxe2x80x9d (5-(i)). The fifth configuration can be combined with at least any one of the first to fourth configurations. The sign condition for the level difference E is defined such that a direction toward the front end of the insulator along the center axis of the insulator is positive. Accordingly, when the front end face of the insulator is located frontward of the rear edge of the end face of the semi-creeping discharge ground electrode, the level difference E assumes a positive value. In the reverse case, the level difference E assumes a negative value.
According to the fifth configuration, a spark directed from the rear edge of the end face of the semi-creeping discharge ground electrode to the center electrode is blocked off by a front end portion of the insulator, and thus the spark does not travel straight from a spark generation position on the semi-creeping discharge ground electrode to the center electrode, but is caused to change traveling directions and to creep along the circumferential surface of the insulator. As a result, the discharge path of spark changes every sparking, and thus the range of creepage of spark on the front end face of the insulator is widened, thereby reducing the amount of channeling and eliminating xe2x80x9ccarbon foulingxe2x80x9d over a wide range of insulator surface through spark-utilized cleaning.
A spark caused to change traveling directions and to creep along the circumferential surface of the insulator involves an elongated discharge path and an increased spark generation voltage. Thus, in order to avoid such sparking, the frequency of sparking from the front edge, rather than from the rear edge, of the end face of the semi-creeping discharge ground electrode tends to increase, a spark from the front edge attacking the insulator more softly. Such a tendency also contributes to suppression of channeling. Sparking from the front edge effectively improves ignition characteristics, thereby effectively suppress misfire and a like problem. Particularly, when the level difference E is small; i.e., a lap along the direction of the center axis between the end face of the semi-creeping discharge ground electrode and the circumferential side surface of the insulator is narrow, sparking from the rear edge of the end face of the semi-creeping discharge ground electrode becomes likely, since sparking distance becomes relatively short. However, by adjusting the level difference E and the radius of curvature R of a curved surface extending from the front end face of the insulator to the circumferential side surface of the insulator so as to establish the relationship (5-(i)), the frequency of sparking from the front edge can be increased, thereby contributing to suppression of channeling or to enhancement of ignition characteristics. The present configuration is particularly effective for a spark plug having a narrow lap; specifically, a level difference E of not greater than 0.5 mm. The lower limit of the E value is appropriately determined such that semi-creeping discharge is not disabled. For example, when the E value is negative as shown in FIG. 4, the E value is determined such that an absolute value thereof becomes smaller than the air gap a.
In the present configuration, the insulator includes a straight tubular portion. Forming an front end portion of the insulator into a straight tubular shape suppresses transmission of heat received by the front end portion in the course of a combustion cycle of an internal combustion engine, to a retainment portion of the insulator retained by the metallic shell, thereby facilitating increase of the front end temperature of the insulator. Therefore, even in a direct-injection-type internal combustion engine, which encounters difficulty in raising the front end temperature of the insulator, there can be facilitated increase of the front end temperature of the insulator, thereby facilitating burning out of an adhering foul substance such as carbon which has been deposited through xe2x80x9ccarbon fouling.xe2x80x9d In such a configuration, since the thermal volume of a front end portion of the insulator is small, the insulator is likely to be cooled by an intake gas of relatively low temperature introduced from an intake pipe. Therefore, the front end temperature of the insulator is unlikely to increase to a level at which preignition occurs in the course of a combustion cycle in an internal combustion engine.
When the rear edge of the end face of the semi-creeping discharge ground electrode is located rearward of the rear end position of the straight tubular portion, dimensional setting of gaps becomes difficult. Therefore, the positional relationship between the straight tubular portion and the semi-creeping discharge ground electrode is set such that the rear edge of the end face of the creeping discharge ground electrode is aligned with or located frontward of the rear end position of the straight tubular portion. When the straight tubular portion becomes too long, a spark generated by the semi-creeping discharge ground electrode tends to be considerably dragged backward along the straight tubular portion, possibly impairing ignition characteristics. The straight tubular portion must be at least 0.5 mm long; otherwise, dimensional setting of gaps becomes difficult, and the above-described effect may not be sufficiently yielded. Preferably, the straight tubular portion is set to a length of 0.5 mm to 1.5 mm.
A spark plug according to a sixth configuration is characterized by assuming the previously described basic structure, and in that:
a projection amount F of the insulator projecting frontward beyond dimension A specified in an applicable JIS Standard (JIS B 8031) or a corresponding ISO Standard (ISO1910, ISO2704, ISO2346, ISO/DIS8479, ISO2705, ISO2344, ISO2345, ISO2347, or ISO3412) comparatively described in the JIS Standard falls within a range of 3.0 mm to 5.0 mm (6-(i)). The sixth configuration can be combined with at least any one of the first to fifth configurations.
According to the sixth configuration, the projection amount F of the insulator is set to a value falling within the range (6-(i)), thereby enhancing ignition characteristics and increasing the front end temperature of the insulator. As compared with an atmosphere around a spark generation position, an atmosphere between the insulator and the front end face of the metallic shell exhibits very low mixture concentration. However, employment of a projection amount F falling within the range (6-(i)) causes increase of voltage required to generate a spark in the atmosphere of low mixture concentration between the insulator and the front end face of the metallic shell, thereby suppressing sparking in the atmosphere. As a result, the range of injection end timing in which no misfire occurs can be extended.
A spark plug according to a seventh configuration is characterized by assuming the previously described basic structure, and in that:
the air gap (xcex1) is not greater than 1.1 mm (7-(i));
the insulator gap (xcex3) falls within a range of 0.5 mm to 0.7 mm (7-(ii));
and the difference "psgr" (unit: mm) between an insulator front-end diameter xcfx86D (unit: mm) and the width of the semi-creeping discharge ground electrode is not greater than 1.8 mm, where, in an orthogonal projection of the insulator onto a virtual plane in parallel with the axis of the insulator, the insulator front-end diameter xcfx86D is defined as the distance between two points of intersection of a first extension line formed through outward extension of a line indicative of the front end face of the insulator and two second extension lines formed through frontward extension of two lines indicative of the circumferential side surface of the insulator located in opposition to each other with respect to the axis of the insulator and facing the semi-creepage gap (xcex2) (7-(iii)). The seventh configuration can be combined with at least any one of the first to sixth configurations.
By reducing the difference "psgr" between the insulator front-end diameter xcfx86D and the width of the semi-creeping discharge ground electrode, a tendency for a spark generated from the semi-creeping discharge ground electrode to be considerably dragged rearward can be prevented. As a result, the range of injection end timing in which no misfire occurs can be extended, and ignition characteristics in a fuel lean state can be enhanced. An increase in the difference causes a spark to considerably detour along the outer circumferential surface of a front end portion of the insulator when sparking occurs between the semi-creeping discharge ground electrode and the center electrode, conceivably for the following reason. When a spark is generated obliquely rearward from a rear corner portion of the end face of the semi-creeping discharge ground electrode, the spark hits against a front end portion of the insulator and then reaches the center electrode. When the spark hits against a front end portion of the insulator, the spark creeps rearward along the outer circumferential surface of the insulator and then changes directions to creep toward the circumferential side surface of the front end of the center electrode. Therefore, if the difference between the insulator front-end diameter and the width of the semi-creeping discharge ground electrode is large, there increases the amount of creepage of a spark when the spark creeps obliquely rearward along the outer circumferential surface of the insulator, with a resultant great rearward drag of the spark.
In order for the difference "psgr" between the width of the semi-creeping discharge ground electrode and the distance between two points of intersection of the first extension line and the two second extension lines to satisfy the relationship (7-(iii)), preferably the insulator front-end wall thickness xcfx81xe2x80x94which is defined as the minimum distance between a point of intersection of the first extension line and the second extension line formed through frontward extension of the line indicative of the circumferential side surface of the insulator facing the semi-creepage gap (xcex2) and a point of intersection of the first extension line and an extension line indicative of the wall of the center through-holexe2x80x94is not greater than 0.9 mm (7-(iv)).
Since satisfaction of the above-described relationship allows reduction of the insulator front-end wall thickness, discharge voltage can be decreased through concentration of electric field intensity, and the amount of channeling can be reduced through suppression of discharge voltage associated with the semi-creepage gap (xcex2). Since the temperature of the front end of the insulator readily increases, in application to a direct-injection-type internal combustion engine, which is prone to carbon fouling, self-cleaning property is enhanced considerably. Since the entire insulator can be thin-walled, a wide clearance can be established between the metallic shell and the insulator, particularly in a spark plug of small diameter. When the wall thickness of the insulator becomes too thin, a spark may penetrate through the insulator at high possibility. Thus, the insulator front-end wall thickness xcfx81 is preferably not less than 0.6 mm, more preferably not less than 0.7 mm.
A spark plug according to an eighth configuration is characterized by assuming the previously described basic structure, and in that:
the projection amount H of the center electrode projecting from the front end face of the insulator is not greater than 1.25 mm (8-(i)). The eighth configuration can be combined with at least any one of the first to seventh configurations.
Particularly in a direct-injection-type internal combustion engine, sparking across the semi-creepage gap (xcex2) during high-speed operation causes narrowing of the range of injection end timing in which no misfire occurs. However, according to the eighth configuration, the projection amount H of the center electrode projecting from the front end face of the insulator is set to the range (8-(i)), thereby allowing further reduction in the distance between the position of the air gap (xcex1), which is a regular spark discharge gap, and a spark generation position associated with the semi-creeping discharge ground electrode. Thus, even in a direct-injection-type internal combustion engine, in which ignition characteristics tends to vary depending on a spark generation position, a spark which is generated from the semi-creeping discharge ground electrode upon occurrence of xe2x80x9ccarbon foulingxe2x80x9d provides sufficient ignition characteristics. Preferably, the projection amount H of the center electrode projecting from the front end face of the insulator is not greater than 0.5 mm. Employment of such H value facilitates dispersion of spark propagation paths around the center electrode, thereby enhancing resistance to channeling and cleaning property for eliminating xe2x80x9ccarbon fouling.xe2x80x9d The H value may be negative; i.e., the center electrode may be retracted from the front end face of the insulator. In this case, an H value not less than xe2x88x920.3 mm is preferred (a depth of recess not greater than 0.3 mm is preferred), in view of further enhancement of resistance to channeling and cleaning property for eliminating xe2x80x9ccarbon fouling.xe2x80x9d
A spark plug according to a ninth configuration is characterized by assuming the previously described basic structure, and in that the air gap (xcex1), the semi-creepage gap (xcex2), and the insulator gap (xcex3) satisfy the relationship xe2x80x9cxcex1xe2x89xa60.4xc3x97(xcex2xe2x88x92xcex3)+xcex3xe2x80x9d (9-(i)). The ninth configuration can be combined with at least any one of the first to eighth configurations.
The air gap (xcex1), the semi-creepage gap (xcex2), and the insulator gap (xcex3) satisfying the relationship (9-(i)) effectively suppresses inverse sparking and associated metallic-shell-insulator sparking. When an ambient gas around a gap of a spark plug is flowing as in application to an actual internal combustion engine, sparking is more likely to occur between the insulator and the front end face of the metallic shell; thus, satisfaction of the relationship (9-(i)) is favorable to suppression of such sparking.
In the ninth configuration, preferably the air gap (xcex1) and the insulator gap (xcex3) satisfy the relationship xe2x80x9c(xcex1xe2x88x92xcex3)xe2x89xa60.4 mm. Satisfaction of the relationship can reduce the amount of channeling in an internal combustion engine involving severe channeling conditions such as an internal combustion engine with a supercharger or an internal combustion engine with high compression ratio. However, when the (xcex1xe2x88x92xcex3) value is less than 0.2 mm, the frequency of discharge induced by the semi-creeping discharge ground electrode decreases, potentially impairing cleaning property for eliminating xe2x80x9ccarbon fouling.xe2x80x9d Therefore, preferably, the (xcex1xe2x88x92xcex3) value is not less than 0.2 mm.
Generally, even when a spark plug is not suffering xe2x80x9ccarbon fouling,xe2x80x9d a spark is not generated only across the air gap (xcex1), but may also be generated across the insulator gap (xcex3). Even when an internal combustion engine is operating under the same conditions, voltage required for initiating sparking differ among gaps of a spark plug, since an ambient atmosphere differs among the gaps. Accordingly, when the air gap (xcex1) is lower in voltage required for sparking than the insulator gap (xcex3), sparking arises across the air gap (xcex1).
Since voltage required for sparking varies in each gap, measurement of minimum and maximum voltages required for sparking reveals that the spans of voltage required for sparking across the air gap (xcex1) and the insulator gap (xcex3) may overlap with each other. The span of overlap substantially depends on the size of the gaps. When discharge voltage required for initiating sparking increases to a level of the overlap according to an ambient atmosphere around a gap of a spark plug, whether sparking is initiated across the air gap (xcex1) or across the insulator gap (xcex3) becomes uncertain. In such a case, sparking across the insulator gap (xcex3) tends to induce channeling because of its high discharge voltage.
When, in order to prevent such channeling, the air gap (xcex1) is narrowed so as to decrease the difference between the air gap (xcex1) and the insulator gap (xcex3), the maximum voltage required for initiating sparking across the air gap (xcex1) decreases, and thus the overlap is narrowed. As a result, unnecessary sparking across the insulator gap (xcex3) can be suppressed, and discharge voltage at the time of sparking across the insulator gap (xcex3) decreases to thereby reduce the amount of channeling. Employment of an air gap (xcex1) not greater than 0.9 mm can suppress voltage required for initiating sparking to a low level, and thus is particularly effective for a high-thermal-value-type plug (a plug whose distance between the front end of an insulator and a retainment portion of the insulator retained by a metallic shell is short), whose insulation resistance between a center electrode and a metallic shell is prone to decrease upon occurrence of xe2x80x9ccarbon fouling.xe2x80x9d
A spark plug according to a tenth configuration is characterized by assuming the previously described basic structure, and in that the width of the semi-creeping discharge ground electrode as viewed from an axially frontward side of the insulator and measured at at least the end face thereof is greater than the diameter of the center through-hole of the insulator as measured at the front end thereof. The tenth configuration can be combined with at least any one of the first to ninth configurations.
According to the tenth configuration, the semi-creeping discharge ground electrode is formed such that the width of the semi-creeping discharge ground electrode as measured at at least the end face thereof is greater than the diameter of the center through-hole of the insulator as measured at the front end thereof (thus is greater than the outside diameter of the front end face of the center electrode or that of the front end face of a noble metal chip, which will be described later). Thus, a spark creeping along the front end face of the insulator covers a wide range of insulator surface, thereby reducing the amount of channeling and eliminating xe2x80x9ccarbon foulingxe2x80x9d over a wide range of insulator surface through spark-utilized cleaning.
A spark plug according to an eleventh configuration is characterized by assuming the previously described basic structure, and in that a front end portion of the insulator is formed into a straight tubular portion having a reduced diameter, and a portion of the insulator located axially rearward of and adjacent to the straight tubular portion is formed into a bulge portion having a diameter greater than that of the straight tubular portion;
the length of the straight tubular portion is not greater than 1.5 mm; and
on a virtual plane including the axis of the insulator and a midpoint of the rear edge, as viewed along the axial direction of the insulator, of the end face of the semi-creeping discharge ground electrode, the bulge portion is located entirely outside a circle with a center thereof at the midpoint of the rear edge and a radius of (xcex3+0.1) mm, where xcex3 (unit: mm) is a distance across the insulator gap. The eleventh configuration can be combined with at least any one of the first to tenth configurations.
The eleventh configuration also employs the straight tubular portion having a length not greater than 1.5 mm (preferably not less than 0.5 mm). The effect of the straight tubular portion is as described above in the section of the fifth configuration. For a structural reason, the bulge portion having a diameter greater than that of the straight tubular portion is formed axially rearward of and adjacent to the straight tubular portion. If the bulge portion is too close to the rear edge of the end face of the semi-creeping discharge ground electrode, a spark from the rear edge tends to be directed toward an electric field concentration part of the bulge portion (particularly an edge of a shoulder portion, the edge being radiused or machined in a like manner) and thus be dragged rearward, potentially impairing ignition characteristics.
In order to cope with the problem, according to the eleventh configuration, on a virtual plane including the axis of the insulator and a midpoint of the rear edge, as viewed along the axial direction of the insulator, of the end face (which serves as a discharge face for discharge across the semi-creepage gap) of the semi-creeping discharge ground electrode, the bulge portion is located entirely outside a circle with a center thereof at the midpoint of the rear edge and a radius of (xcex3+0.1) mm, where xcex3 (unit: mm) is a distance across the insulator gap. In this manner, the bulge portion is located away from the rear edge of the end face of the semi-creeping discharge ground electrode, thereby effectively suppressing a rearward drag on a spark from the semi-creeping discharge ground electrode and thus maintaining good ignition characteristics.
A spark plug according to a twelfth configuration is characterized by assuming the previously described basic structure, and in that the diameter of the center through-hole of the insulator is reduced at a front end portion of the insulator. The twelfth configuration can be combined with at least any one of the first to eleventh configurations. Since a spark plug of the present invention includes semi-creeping discharge ground electrodes, the twelfth configuration appropriately suppress a tendency for heat received by a front end portion of the insulator in the course of a combustion cycle in an internal combustion engine to be released to the center electrode, thereby facilitating increase of the front end temperature of the insulator. Therefore, even in a direct-injection-type internal combustion engine, which encounters difficulty in raising the front end temperature of the insulator during regular operation, there can be facilitated increase of the front end temperature of the insulator, thereby facilitating burning out of carbon which has been deposited through xe2x80x9ccarbon fouling.xe2x80x9d This prevents generation of a spark between the insulator and the front end face of the metallic shell and generation of a spark in the vicinity of a retainment portion. Thus, even in application to a direct-injection-type internal combustion engine, the range of stable combustion can be widened. Preferably, the twelfth configuration satisfies Additional Requirement 3, which will be described later.
A spark plug according to a thirteenth configuration is characterized by assuming the previously described basic structure, and in that, with the term xe2x80x9cfrontwardxe2x80x9d referring to a side toward the front end portion of the insulator along the axial direction of the insulator and with a plane of projection being defined as a plane including the axis of the insulator and perpendicularly intersecting a virtual plane including the axis of the insulator and a midpoint of the rear edge of the end face of the semi-creeping discharge ground electrode, the end face as orthogonally projected on the plane of projection is shaped such that, on the plane of projection, with X referring to a point of intersection of the axis and the rear edge, Y referring to a point of intersection of the axis and the front edge, and a reference line being defined as a line passing through a midpoint of segment XY and perpendicularly intersecting the axis, area S1 of a domain located frontward of the reference line is greater than area S2 of a domain located rearward of the reference line. The thirteenth configuration can be combined with at least any one of the first to twelfth configurations.
In view of suppression of channeling and enhancement of ignition characteristics, preferably sparking from the semi-creeping discharge ground electrode is such that, on the end face thereof serving as a discharge face, the frequency of sparking from the front edge rather than from the rear edge is increased, since a spark from the front edge attacks the insulator more softly. Thus, the end face of the semi-creeping discharge ground electrode is shaped such that area S1 of a domain located frontward of the reference line, which is located at an intermediate position between the front edge and the rear edge, is greater than area S2 of a domain located rearward of the reference line, thereby increasing the frequency of sparking from the front edge of the end face and thus contributing to suppression of channeling or to enhancement of ignition characteristics.
A spark plug according to a fourteenth configuration is characterized in that, with the term xe2x80x9cfrontwardxe2x80x9d referring to a side toward the front end portion of the insulator along the axial direction of the insulator and with a plane of projection being defined as a plane including the axis of the insulator and perpendicularly intersecting a virtual plane including the axis of the insulator and a midpoint of the rear edge of the end face of the semi-creeping discharge ground electrode, the end face as orthogonally projected on the plane of projection is shaped such that, on the plane of projection, with X referring to a point of intersection of the axis and the rear edge, Y referring to a point of intersection of the axis and the front edge, and a reference line being defined as a line passing through a midpoint of segment XY and perpendicularly intersecting the axis, at least a corner portion of a domain located rearward of the reference line is radiused at a radius of curvature of not less than 0.2 mm or chamfered at a width of not less than 0.2 mm, or two sides defining the corner portion form an angle greater than 90 degrees. The fourteenth configuration can be combined with at least any one of the first to thirteenth configurations.
The gist of the fourteenth configuration is to suppress sparking from the rear edge of the end face, which serves as a discharge face, of the semi-creeping discharge ground electrode. When a sharp corner portion is present, the portion tends to serve as a starting point of sparking. Elimination of such a sharp corner portion from the domain located rearward of the reference line suppresses sparking from the rear edge of the end face. As a result, the frequency of sparking from the front edge can be increased, thereby contributing to suppression of channeling or to enhancement of ignition characteristics. When such a sharp corner portion is formed at each end of the rear edge, the sharp corner portions may serve as starting points of sparking such that sparks from the corner portions are obliquely dragged backward to a great extent, potentially resulting in significantly impaired ignition characteristics. The fourteenth configuration eliminates a sharp corner portion from the rear edge, thereby preventing or suppressing such a problem. Combination of the fourteenth configuration with the thirteenth configuration suppresses channeling or enhances ignition characteristics far more effectively.
Next will be described additional requirements, which are common among spark plugs according to the above-described first to fourteenth configurations (including combination thereof).
(Additional Requirement 1)
A front end portion of the insulator can be formed into a straight tubular portion such that the straight tubular portion extends rearward of the front end face of the metallic shell. This configuration allows establishment of a further increased diametral difference between the insulator and the front end face of the metallic shell, thereby facilitating suppression of sparking at the position of the front end face. Preferably, the length of the straight tubular portion is up to 1.5 mm. Action and effect in relation to formation of the straight tubular portion are similar to those described in the section of the eleventh configuration.
(Additional Requirement 2)
A noble metal chip formed of a noble metal or noble metal alloy having a melting point not lower than 1600xc2x0 C. can be joined to a front end portion of a base material of the center electrode. In this case, a joint of the chip and the base material is located within the center through-hole. Disposing the joint inside the center through-hole allows sparking between the semi-creeping discharge ground electrode and the noble metal chip not only when sparking arises across the air gap (xcex1) but also when sparking arises across the semi-creepage gap (xcex2). Accordingly, durability is enhanced regardless of whether sparking arises across either gap. In addition to Pt and Ir, noble metal alloys having a melting point not lower than 1600xc2x0 C. such as Pt alloys and Ir alloys; specifically, Ptxe2x80x94Ir, Irxe2x80x94Rh, Irxe2x80x94Pt, and Irxe2x80x94Y2O3, are preferred.
(Additional Requirement 3)
Preferably, the minimum bore diameter (D3) of the center through-hole as measured at a front end portion of the insulator located frontward of a retainment portion of the metallic shell, the insulator being engaged with and retained by the retainment portion, is not greater than 2.1 mm. Such reduction of the bore diameter of the insulator allows reduction of the outside diameter of the center electrode. Thus, heat received by a front end portion of the insulator in the course of a combustion cycle in an internal combustion engine encounters some difficulty in being released toward the center electrode, thereby facilitating increase of the front end temperature of the insulator. Therefore, even in a direct-injection-type internal combustion engine, which encounters difficulty in raising the front end temperature of the insulator during regular operation, there can be facilitated increase of the front end temperature of the insulator, thereby facilitating burning out of carbon which has been deposited through xe2x80x9ccarbon fouling.xe2x80x9d This prevents generation of a spark between the insulator and the front end face of the metallic shell and generation of a spark in the vicinity of the retainment portion. Thus, even in application to a direct-injection-type internal combustion engine, the range of stable combustion can be widened. However, in view of prevention of channeling, the D3 value is preferably not less than 0.8 mm.
(Additional Requirement 4)
When the above-mentioned noble metal chip is to be used, the noble metal chip can be configured such that the outside diameter of a joint portion between the noble metal chip and the base material of the center electrode is greater than that of a front end portion used to define the air gap (xcex1). Through employment of such a configuration, even when a spark is generated across the semi-creepage gap (xcex2), dropping off of the noble metal chip from the base material of the center electrode can be prevented. Specifically, when sparking arises across the semi-creepage gap (xcex2), a spark is generated between the side surface of the noble metal chip and the semi-creeping discharge ground electrode. Frequent sparking at this position causes the noble metal chip to be ablated in the vicinity of the front end face of the insulator; as a result, the diameter of the ablated portion becomes smaller than that of a front end portion of the noble metal chip. Thus, sparking across the semi-creepage gap (xcex2) is repeated, a front end portion of the noble metal chip may finally drop off. However, through increase of the diameter of a joint portion of the chip, such a phenomenon can be suppressed.
Since the diameter of a front end portion of the noble metal chip is smaller than that of a joint portion of the chip, discharge voltage at the time of sparking across the air gap (xcex1) can be decreased, thereby enhancing ignition characteristics. Particularly, in application to a direct-injection-type internal combustion engine, the range of stable combustion can be widened. A diametrally enlarged portion of the noble metal chip may be retracted inward from the front end face of the insulator. In this case, when a spark is generated across the semi-creepage gap (xcex2), the spark creeps along the front end face of the insulator and then along the inner wall of the center through-hole of the insulator, and then reaches the diametrally enlarged portion of the noble metal chip. Therefore, even though the diametrally enlarged portion is located within the center through-hole of the insulator, a spark is generated between the diametrally enlarged portion and the semi-creeping discharge ground electrode, thereby yielding the above-described effect.
(Additional Requirement 5)
The minimum diametral difference between the outside diameter of the noble metal chip and the bore diameter of the center through-hole can be not greater than 0.2 mm. This facilitates suppression of ablation of the base material of the center electrode, the ablation potentially being induced by spark discharge. As described above, when a spark is generated across the semi-creepage gap (xcex2), the spark creeps along the inner wall of the center through-hole of the insulator. At this time, if the diametral difference between the outside diameter of the noble metal chip and the bore diameter of the center through-hole of the insulator is large, the spark may not head for the noble metal chip, but may creep deep into the center through-hole up to the base material of the center electrode. The base material of the center electrode is lower in spark ablation resistance than the noble metal chip, and thus is prone to quick ablation, potentially resulting in dropping off of the chip. Therefore, reduction of the diametral difference suppresses a phenomenon that a spark reaches the base material of the center electrode, thereby enhancing durability. Notably, herein the expression xe2x80x9cminimum diametral differencexe2x80x9d represents the following diametral difference. When the outside diameter of the noble metal chip and the bore diameter of the center through-hole are uniform along the axial direction, the diametral difference becomes substantially uniform along the axial direction. However, when either the outside diameter of the noble metal chip or the bore diameter of the center through-hole is not uniform along the axial direction (for example, when the center through-hole is slightly tapered), the minimum diametral difference as measured along the axial direction is employed as a representative value.